Hyperbolic plasmon modes in tilted Dirac cone phases of borophene.

Hyperbolic materials are receiving significant attention due to their ability to support electromagnetic fields with arbitrarily high momenta and, hence, to achieve very strong light confinement. Here, based on first-principles calculations and many-body perturbation theory, we explore the characteristic of two-dimensional plasmon modes and its hyperbolic properties for two phases of single layer boron hosting tilted Dirac cone, namely, the $hr$-$sB$ and $8Pmmn$ borophene. In-plane anisotropy in borophene is manifested in the structural, electronic, vibrational and optical properties. We find two hyperbolic regimes for both phases of borophene, where the high-energy one is located in the visible range. The $hr$-$sB$ borophene is characterised with an intrinsic high carrier density and it supports strong hyperbolic plasmon modes in the visible part of the spectrum. The $8Pmmn$ borophene, on the other hand, resembles the prototypical Dirac material graphene, and upon carrier doping acquires anisotropic Dirac plasmons in the mid-infrared. We have also investigated the impact of the electron-phonon coupling and Landau damping on these hyperbolic plasmon modes. Our results show that borophene, having high anisotropy, intrinsic high carrier concentration, low-loss hyperbolic Dirac plasmon modes, and high confinement can represent a promising candidate for low-loss broad band surface plasmon polariton devices.